The objective of this proposal is a quantitative study of urea production and excretion by the sheep fetus, normally and during maternal starvation. Placental urea and glucose clearances will be measured simultaneously at different gestational ages. Purpose of these measurements is to observe changes in these clearances during gestation, whether the two clearances vary in a constant ratio and whether fetal growth retardation is associated with abnormally low values of placental glucose clearance per Kg of fetus.